1. Field of the Invention
This invention relates to an ion sensor suited for the analysis of ions in a biological fluid/material. More specifically, the invention relates to an ion sensor suited for the analysis for potentiometrically measuring ions and to a clinical analyzer.
2. Description of Related Art
Ion sensors have a feature of selectively determining a particular ion concentration in the solution and have heretofore been used in a wide field of applications such as monitoring the concentration of particular ions and analyzing the water. In the field of medical treatment, in particular, the ion sensors have been used for determining ions contained in the biological fluid/material such as blood and urine, e.g., for determining chloride ions and potassium ions. This is based on that the concentration of particular ions in the biological fluid/material has an intimate relationship to the metabolic reaction of the biological fluid/material. Upon measuring the ion concentrations, various diseases such as hypertension, kidney disorder, nervous disorder and the like diseases can be diagnosed. Between the ionic activity a treated by the ion sensor and the potential E exhibited by the ion sensor, there holds a relationship expressed by,E=E0+2.303(RT/ZF) log ain which the logarithm of the activity changes in proportion to the potential, and from which a desired ionic activity can be easily calculated from the potential that is measured. In the above formula, R is a gas constant, T is an absolute temperature, Z is a valency, F is a Faraday constant, and E0 is a standard electrode potential of the system. Use of this ion sensor makes it possible to determine the ions over a wide range of concentrations by simply measuring the potential.
The chloride ion sensor chiefly uses a quaternary ammonium salt as an tonically sensitive substance, and study has been conducted extensively in an effort to improve the selectivity (Mikrochimica Acta [Wien], 1984, III, 1–16). Japanese Patent Application No.23151/1989 teaches a sensor featuring excellent selectivity, in which a tetraoctadecylammonium salt is carried as a sensitive substance by a high-molecular support film such as of polyvinyl chloride. In the sensor using an ion sensitive liquid membrane, water is permitted to pass through little due to the use of an ester, an alcohol and a polyvinyl chloride having a high oleophilic property as a main component of a membrane, the water migrates little in the inner solution, and the composition of the inner solution remains stable. There, however, remains a problem in that the slope sensitivity of the electrode loses stability due to the elution of the sensitive substance and of the plasticizer. There have further been known sensors using an ion-exchange membrane in which a quaternary ammonium salt is fixed to a polymer matrix as taught in JP-A-57-40642 and JP-B-2-13262. It is considered that the slope sensitivity of the electrode remains highly stable since the sensitive substance has been fixed and elutes out very little. However, the ion-exchange membrane has been developed in order to remove the salt and, hence, permits ions to easily pass through and, accordingly, permits the water to easily pass through. If the ion-exchange membrane is used as the sensitive film of the ion sensor, there arouses a problem in that water easily migrates in the internal solution whereby the internal solution is depleted and the sensor no longer works. Therefore, there has been reported a method in which a member for suppressing the permeation of water is provided neighboring the ion-exchange membrane as taught in JP-A-2000-28568. As a member for suppressing the permeation of water, there has been used a material that permits water to pass through little but permits ions to pass through easily. Concretely speaking, there have been taught (1) a plasticized polymer membrane in which a quaternary ammonium salt and a plasticizer are dispersed in a polyvinyl chloride, (2) a plasticized polymer membrane in which a quaternary ammonium salt, a crown ether and a plasticizer are dispersed in a polyvinyl chloride, and (3) a polymer membrane in which a polypyrrole film is doped with iodine ions. The polymer membrane is adhered by using an organic solvent to the surface of the ion-exchange membrane on the side opposite to the side that comes in contact with the sample. The sensor incorporating the ion-exchange membrane integrally formed with the water permeation-suppressing member formed by those methods, suppresses a drop in the slope sensitivity even after preserved for three months.